In a trawl fishing method using a trawl net, appropriate spreading of the trawl net is important from the perspective of increasing exploitation rate and maintaining the quality of fishes that are caught. Therefore, since the past, an otter board is used as a means for spreading the trawl net in the trawl fishing method.
In general, the otter board is disposed in a position between a trawler and a trawl net. The otter board is towed by the trawler in a state in which the otter board is connected to the trawler with a towing rope (warp) set out from the trawler therebetween, and connected to the trawl net with an otter pennant therebetween. In accompaniment with the otter board being towed, a net opening of the trawl net is expanded.
Here, since the past, in the otter board, a vortex referred to as a wingtip vortex generated in an end section of the otter board during towing and an increase in resistance obstructing towing of the trawl net as a result of separation of flow at a front edge of a back surface of the otter board have been considered problems.
To solve the above-described problems, otter boards such as those described in Patent Literature 1 and Patent Literature 2 have been proposed in the past.
In other words, in Patent Literature 1, reduction of vortexes is attempted by flow boards being provided in the upper and lower ends of a main body of an otter board.
In Patent Literature 2, reduction of vortexes is attempted by a rectifying fin being provided on a front edge of a back surface of a main body of a vertical otter board, the rectifying fin successively widening towards the rear of the otter board main body in a traveling direction.    Patent Literature 1: Japanese Utility Model Laid-open Publication No. Showa 50-71489    Patent Literature 2: Japanese Utility Model Laid-open Publication No. Showa 61-193766